In recent years, there has been a development in wireless communication technology and an increase in the number of subscribers leading to a substantial increase in data traffic. Furthermore, new applications such as gaming, music downloads, video transmission etc require an increase in transmission bandwidth. One technology used for wireless data transmission is WCDMA.
WCDMA—Wideband Code Division Multiple Access—was developed, at least partly, to ensure global roaming capabilities by setting a global standard for real-time multimedia services. The standard was initially supported by the International Telecommunication Union, ITU, and was later taken over by 3rd Generation Partnership Project, 3GPP.
The general idea is to allow multiple terminals to share a common channel. There are several widely used techniques, including Frequency Division Duplex or Frequency Division Multiplex, Time Division Duplex or Time Division Multiplex and Code Division Multiplex. For multiple access techniques include TDMA, FDMA and CDMA, i.e. Code Division Multiple Access, CDMA and Wideband Code Division Multiple Access, WCDMA. The multiple access methods allow resources to be divided between different users, the duplex/multiplex methods are used to divide resources to provide two-way communication. The methods may be combined, e.g. in GSM where TDMA is mixed with FDMA so that users are allocated different timeslots and in order to accommodate more users, FDMA is used, allowing several frequencies to be used.
In Frequency Division Duplex, a frequency is divided into a number of sub-bands where each terminal is allocated a given sub-band. This requires good frequency filtering. Examples of technology using Frequency Division Duplex include HSPA/HSDPA, High Speed Packet Access/High Speed Downlink Packet Access.
In Time Division Duplex, one channel is divided into a number of time slots where a given unit is allowed to transmit in a limited number of time slots. This requires that all terminals are synchronized. Examples of systems using Time Division Duplex include WiMAX and TDSCDMA.
In CDMA systems all users, i.e. terminals, transmit within the same bandwidth simultaneously. The individual data is related to a specific user via codes in the data.
In uplink multiplexing today, each terminal has its own non-orthogonal scrambling code.
In case of synchronous transmission, several terminals may share a common scrambling code and terminal multiplexing will be carried out through assigning different orthogonal spreading codes to different terminals. To share a common scrambling code the terminals are required to be synchronized. To maintain the orthogonality in the uplink, all (active) terminals need to be synchronized in the sense that the signals from different terminals should reach the base station at the same time. Similarly to the downlink, it can be expected that different channels belonging to the same terminal are also code multiplexed. Hence, data and control signaling may be allocated to different spreading codes and transmitted simultaneously.
One problem that will occur when several terminals share the same scrambling code in the uplink is that the system may become code limited. Each terminal or user needs a number of spreading codes for control purposes. This will happen regardless if the terminal has any data to transmit or not. Also, for simplifying signaling it is likely that a fixed amount of the now common code resource has to be allocated for this use. Hence, supporting a large number of users will require a large amount of the common code resource just for control purposes and fewer codes will be available for data transmission.
US Patent publication No. 2007/0104085 discloses a method of varying scrambling code in which a scrambling code can be allocated to a user device in dependence of detected interference. Two different scrambling code sequence sets may be employed for increasing variation of scrambling codes to increase interference diversity and reduce a jamming effect caused bo a dominant interferer in a sector near an interfered user's sector.
Mogensen, P. E. et al.: “Capacity Gain of an Uplink-Synchronous WCDMA System Under Channelization Code Constraints” IEEE Transactions on Vehicular Technology, IEEE Service Center, Piscataway, N.J., US, vol. 53, No. 4, 1 Jul. 2004 (Jul. 1, 2004), pages 982-991, XP011115281, ISSN 00188-9545 discloses the use of more than one set of scrambling codes to increase the number of available scrambling codes.
It is an object of the present invention to provide a method and system to provide improved support for multiple terminals.